Effects of host plant inter-and intra-specific variation of cotton, soybean and clover on the yield and infectivity of occlusion bodies (OBs) of Helicoverpa zea nucleopolyhedrovirus (HzNPV) in tobacco budworm, Heliothis virescens (F.) and bollworm, H. zea (Boddie) larvae were studied. The number of OBs produced by H. zea or H. virescens larvae fed on vegetative tissues was significantly (P<0.05) higher than by larvae fed on reproductive tissues in all tested plant species. The total number of OBs produced by H. zea or H. virescens larvae on vegetative or reproductive tissues of cotton, soybean and clover varied significantly. LC₅₀ values for both species fed HzNPV were higher when the virus was produced in larvae fed on reproductive tissues than on vegetative tissues on all host plants for H. virescens but only for soybean in H. zea. Thus the infectivity of the OBs produced by H. zea and H. virescens on reproductive tissues was reduced compared to OBs produced in larvae on vegetative tissues. This reduction in infectivity of OBs in larvae fed on reproductive tissues was most prominent in H. virescens on cotton, followed by soybean and clover. In conclusion, viral production and viral infectivity in heliothine larvae was affected by both tissue and plant species.